A. FIELD OF THE INVENTION:
The present invention relates generally to the field of underwater seismic surveying and more particularly to a method and apparatus for maintaining a tuned array of airgun seismic energy sources at a uniform depth below the water's surface without the use of flotation devices.
B. DESCRIPTION OF THE PRIOR ART:
One commonly used source of seismic energy in underwater seismic surveying is the airgun. One airgun is described generally in U.S. Pat. Nos. 3,249,177 and 3,379,273. Airguns of different designs are described in U.S. Pat. Nos. 3,638,752 and 4,285,415.
In general terms, airguns operate by releasing rapidly into the water a charge of high pressure air. Typically, the air is released at a pressure in the order of 2,000 psig over a period of less than one millisecond to less than five milliseconds. However, some air guns operate at pressures as high as 4,500 psig.
The initial release of stored high pressure air into the water produces a primary energy pulse which travels through the water and into the earth. Portions of the energy of the primary pulse are reflected by different earth formations and the reflected energy is recorded through hydrophones.
However, in addition to the primary pulse, the gun also produces a series of secondary or bubble pulses. The bubble pulses occur because the bubble of air produced by the gun oscillates. The bubble pulses have decreasingly smaller amplitudes than the primary pulse, but they tend to obscure or obliterate the primary pulse reflections.
There have been attempts to suppress or eliminate the bubble pulses associated with air guns. One such attempt is known as the wave shape kit technique, which is disclosed in U.S. Pat. No. 3,653,460. The wave shape kit technique involves injecting air into the bubble after the primary pulse thereby to attempt to reduce the magnitude of the implosion of the bubble after the primary pulse. An article entitled "An Efficient Method of Operating the Air-Gun" by M. H. Safar, Volume 28, Geophysical Prospecting, Pages 85-94 (1980), suggests a technique for generating a seismic pulse from the bubble pulses that are radiated by the air gun. The technique is implemented by firing a single air gun several times at the same depth, but with different chamber pressures.
The most common and widely used method for suppressing the energy due to bubble pulses is by the use of what is known as a "tuned array". A tuned array consists of a plurality of airguns that are arranged such that when they are fired the primary pulses interfere constructively and the bubble pulses interfere destructively. The formation of the array involves setting up the individual guns of the array so as to produce bubble pulses of different periods.
One method of forming a tuned array is described in an article entitled "Signature and Amplitude of Linear Air Gun Arrays", by J. J. Nooteboom, Volume 26, Geophysical Prospecting, Pages 194-201 (1978). According to theory developed by United Geophysical Corporation in 1968, the period or "bubble time" can be determined by the following equation: ##EQU1## Where T.sub.6 equals bubble time
P.sup.w equals density of water PA1 P equals compressed air pressure PA1 V equals compressed air volume PA1 P.sub.s equals static water pressure PA1 C equals constant depending on gun design. PA1 P.sub.1 equals air pressure PA1 P.sub.5 equals static water pressure PA1 V.sub.1 equals volume of the bigger gun.
Since static water pressure is a linear function of depth EQU P.sub.s =P.sub.w (D+K)
(where k is a constant dependent on atmospheric pressure) bubble time may be expressed in terms of depth as follows: ##EQU2##
Assuming that the compressed air pressure and depth of the guns in the array are constant, then the bubble time of each gun of the array is proportional to the cube root of the volume of the respective gun EQU T.sub.b .alpha.V1/3
Again, according to theory, which is verified by experiment, the peak amplitude of the pressure wave is also proportional to the cube root of the volume, EQU A.alpha.V1/3
Combining the above two relations shows that the amplitude is proportional to bubble time. Given the foregoing and the fact that the width of the bubble is proportional to bubble time squared, one can select the air volumes of the individual guns so as to form a tuned array. Of course, it will be recognized that volume may be maintained constant and pressure varied or both pressure and volume may be varied in order to form a tuned array.
Preferably, the guns of the tuned array are spaced apart so as to be non-interacting, which means that the guns and their respective bubbles are not substantially affected by the pressure fields of nearby guns. The reason that non-interacting arrays are preferred, follows from the fact that the amplitude is proportional to the cube root of the gun volume. When bubbles from multiple airguns coalesce, the amplitude increases only as the cube root of the sum of their volumes. However, when the air gun spacing is sufficiently great that the guns are substantially non-interacting, their amplitudes are directly additive. Accordin to Nooteboom, the distance necessary to prevent substantial interaction between two guns of Volumes V.sub.1 and V.sub.2 may be expressed as ##EQU3## where D equals distance between guns
However, a paper entitled "Desired Seismic Characteristics of an Air Gun Source" presented at the 48th Annual Meeting of the Society of Exploration Geophysicists in San Francisco, Calif, Oct. 29-Nov. 2, 1978, discloses a tuned array formed with interacting airguns.
From the foregoing, it is clear that in order for the array to be properly tuned, the air guns must be maintained at a uniform depth below the water's surface. Commonly, flotation devices are used to maintain such depth. The flotation devices may be Norwegian buoys or rigid floats. Examples of tuned arrays are disclosed in U.S. Pat. Nos. 3,602,878 and 3,893,559.
There are a number of disadvantages to using flotation devices. For example, the use of rigid floats necessitates a large stern area for the survey vessel and, since a number of guns are hung below the float, that number of guns must be turned off and brought aboard if just one gun fails. Depending on the deploying system, the use of Norwegian buoys may also necessitate the turning off of a number of air guns when one needs to be brought aboard to be repaired.
The interconnected floats, airguns, cables, air lines and electrical lines tend to become snarled and tangled, and are awkward to handle in use and are heavy and very difficult to hoist onboard the vessel when not in use and are difficult to launch into the water in unsnarled condition. Because of the awkwardness and difficulties of handling such complex arrays of equipment at sea under often times difficult conditions, snarl ups and breakages are inevitable.
In U.S. Pat. No. 4,038,630, there is disclosed an air gun marine seismic survey streamer. The streamer includes a plurality of air guns arranged end to end to form an array. The buoyancy of the streamer is controllable so as to maintain the streamer at substantially uniform depth. Each air gun has a generally cylindrical configuration and is of the same diameter, same length and same weight, but its firing chamber volume can be changed and preset to various sizes as may be described by the user. The streamer may be towed directly from a survey vessel without the use of buoys or the like. The buoyance compensating mechanism makes the streamer of the U.S. Pat. No. 4,038,630 complicated.
It is therefore an object of the present invention to provide a tuned underwater seismic energy source array that overcomes the shortcomings of the prior art. More specifically, it is an object of the present invention to provide a tuned underwater seismic energy source array that does not include floats, buoys or buoyancy compensating means.